Produced water
- a North Sea environmental challenge
Anne Berit Bjørken
SINTEF and NTH start research programme and build a new test rig
By the end of the century, discharges of produced water from North Sea oil-fields will have increased by a factor of six. The more serious discharges of oil, chemicals, heavy metals and compounds such as phenols and aromatics, will increase correspondingly.
The authorities, researchers and oil companies are not certain whether these discharges will lead to environmental problems for the North Sea, and if they do, how important they will be. But they do agree that quantities of produced water will greatly increase, and that technology capable of treating produced water of dissolved components is not currently available. For this reason, SINTEF and NTH are in the process of setting up a research programme in this area. It aims both to develop known treatment methods and to develop new technologies which will be ready for use when required and when discharge regulations are adopted. As part of the programme, a test rig will be built to test, adapt and optimize different treatment technologies, singly and in combination, before they are introduced by the oil industry.
"Produced water" is water brought up from the production well together with the oil. Some of it comes from the reservoir, but most of it is water that has been injected in order to recover more oil from the field. As reservoirs gradually empty, more water has to be used to bring up the oil. Aging fields, and the rising rate of hydrocarbon extraction in general, are thus producing a dramatic increase in emissions to the North Sea. At present, most of the oil is removed before the water is released into the sea. Dissolved substances, however, are not removed.
What does produced water contain?
Produced water contains a large number of substances that include "dissolved" and undissolved oil, heavy metals, aromatics, phenols, chemicals, salts and minerals. Some of these come from the reservoirs, where the water loosens various materials from the rock, and some from the chemicals that are added to the water before it is injected. Chemicals have to be added for a number of reasons, e.g. to prevent icing up of the pipe-string, make purification of the injection water more efficient, prevent scaling or blockage of the pores that hold the oil, and prevent corrosion of the pipeline network.A study carried out by Kværner as part of an environmental study performed by the National Association of Oil Companies (OLF), estimated that releases of production chemicals from a typical Norwegian oil field are around 350 kg per day, plus 72 kg/day of heavy metals. As a rough comparison, every day about 60 kg of heavy metals enter VEAS, Norway´s biggest waste treatment plant at Slemmestad near Oslo; this deals with waste from more than 500,000 persons. Of that total, around 30 kg are released to the Oslo Fjord. Releases of organic matter (COD) from a typical oil field come to around 2,300 tons a year. By the turn of the century, the North Sea will receive emissions which are equivalent to 12 such fields, so that annual releases of organic materials will come to a total of 27,000 tons, in addition to 315 tons of heavy metals.
This means that by the year 2,000, produced water from North Sea platforms will release heavy metals equivalent to the raw sewage produced by more than five million persons (or person equivalents: PEs) a year.
Releases of mercury, zinc, chromium and copper will be the equivalent to 3 - 4 million PEs, while nickel, cadmium and lead will come to 22, 29 and 57 million PEs respectively.
"These are large numbers, but we don´t know how dangerous these discharges are," says dr.ing. Bjørnar Eikebrokk, head of research at SINTEF NHL. "These compounds are quickly diluted, and many of them are degradable. But we are uncertain of the long-term effects of some of them. There are great differences in emissions from field to field. Produced water usually contains heavy metals such as lead, cadmium, nickel, zinc and copper in much higher concentrations than seawater" says Eikebrokk.
Purification requirements
At present, the authorities allow produced water to contain a maximum of 40 mg. oil per litre. At present, this threshold is surpassed by a good margin, by treating the water in hydrocyclones or centrifuges before it is released, so that large oil-droplets are separated out and removed. At present there are no discharge regulations regarding heavy metals, chemicals or other dissolved components."The Norwegian authorities are extremely worried, and are looking seriously at the prospect of major increases in quantities of produced water from North Sea oil wells," says Ingunn Valvatne, departmental manager at the State Pollution Control Authority.
"Emissions of oil from this source will increase drastically unless something is done to improve the technology. We realise that activity in the North Sea is going to result in greater emissions of chemicals. Our strategy is to encourage the use of new chemicals that have better environmental characteristics, and to make sure that the oil companies use the most environmentally acceptable chemicals," says Valvatne.
Current technology
Six times as much produced water as at present will have important consequences, not only for the marine environment, but also for the economics of the fields involved. Even today, the water treatment plants that remove the oil have been identified as bottlenecks in the production process. The aim of the SINTEF/NTH research programme is that the technology developed during the programme should reduce environmental loads and be cost-effective. In principle, there are three ways of affecting the dispersed oil fractions, dissolved components and heavy metals that accompany produced water:- improved treatment processes before discharge to the sea
- treatment and reinjection of the produced water
- limiting the quantity of produced water.
No currently available technology is capable of removing all types of dissolved components by means of a single process; two or more different processes must always be run in series. None of the technologies evaluated has yet been employed on offshore installations, and large-scale pilot testing will be needed. Operating and maintenance costs are expected to be high because of the complexity of the processes involved.
Scientists at NTH and SINTEF are currently working on a number of different technologies for treating produced water.
"We are planning our research in close collaboration with NTH. A doctoral study which will evaluate different treatment methods has already been identified; this will emphasize the destabilization and removal of fine particulate substances. I believe that oil companies will also be positive to the idea of strengthening research and teaching in this area," says Eikebrokk.
Effects of produced water
At IKU Petroleum Research in the SINTEF Group, scientists are modelling the dilution and dispersal of produced water. The aim of these studies is to find out how substances disperse and how hazardous they can be. Our calculations start at the end of the outlet pipe and include not only dilution but also what happens to individual components in the sea; for example the evaporation of dissolved components and the natural breakdown of substances in the water."IKU has an environmental modelling group that is currently working on the dispersion and effects of produced water for Statoil and other oil companies. There are two parts to this project. In the first place, the scientists are looking at the spread and dilution of different compounds in the water column. Even though concentrations in the emission itself may be low, very low concentrations are also of interest, because certain special components can reach higher concentrations in the food chain," says Henrik Rye, a scientist at IKU Petroleum Research.
This is what the IKU scientists are looking at in the other project. Given a concentration of a particular substance in the sea, we can model how concentrations of this substances can accumulate, for example in fish. Such concentrations can reach significant levels, and this is why low concentrations in seawater are also of interest.
"This means that very large areas have to be included in our calculations. The project is part of a major research programme that Statoil and other oil companies have launched in order to shed as much light as possible on the potential effects of produced water," says Rye.
Small pores solve big problems
A purification project carried out by SINTEF Materials Technology and Betex A/S for Phillips Petroleum and the former research council NTNF has shown that ceramic membranes can reduce the amount of oil in produced water to less than three milligrams per litre. This is less than 10% of the current upper limit of 40 mg./l. The project has also developed a filter unit known as Filser, which is now on commercial sale."Porous ceramic membranes have a large cleansing effect because the size of the pores can be adapted to the liquid that is to be purified. They are also chemically and thermally resistant and can withstand a great deal of wear from sand and other types of particles. They are easy to clean and are suitable for filtering water that contains oil," says Rune Bredesen, senior scientist at SINTEF Materials Technology.
A ceramic membrane consists of a porous tube with a very thin layer of small pores on the inside of the tube wall. This layer is the actual membrane, and what captures oil and particles when water is forced through it. Ceramic membrane purification is a young technology that is being adopted in more and more areas. SINTEF Materials Technology is working both on the development and testing of ceramic membranes, and they have to their disposal a well equipped lab. for testing liquid filtrations and gas separation for Norwegian industry. Ceramic membranes with pores as small as 0.000003 mm are currently available, and intense efforts are being made to develop membranes with even smaller pores.
Bjørn Christiansen of SINTEF Fluid Machinery.
Test rigs
A multi-purpose test rig for model testing of various pieces of equipment related to oil/gas/water separation has been built by NTH/SINTEF."In the first period, the rig will be used to test non-rotating centrifugal separators that exploit the same phenomena as we observe in a tornado, or when we empty the bath," says Bjørn Christiansen, a scientist at SINTEF Fluid Machinery. Equipment of this type will allow a centrifugal force several hundreds times as strong as the force of gravity to be applied to the well flow, so that the equipment can be made more compact and cheaper than the traditional gravitation separators in use today.
"In a joint project with Kværner Process Systems a.s, we are presently developing a compact centrifugal separation system that removes water from the well flow prior to multiphase transportation from a satellite field. The system should be placed upstream the choke valve on the wellhead, either on the seabed or on unmanned platforms."
The centrifugal separation system consist of three stages, where the first stage separates the gas from the liquid and the second stage separates water from oil, while a final stage purifies the water to meet the standards for emission to the sea.
"Our project is first and foremost financially motivated, but it also produces clear environmental benefits. This technology will make it possible to reduce the amount of chemicals used to prevent corrosion and icing in multiphase pipelines," says Christiansen.
Polymeric membranes and active charcoal
SINTEF Applied Chemistry is setting up a project that aims to develop new technology which will be capable of dealing with all dissolved components in produced water."We will be looking at a combination of polymeric membranes and active charcoal as a means of removing all substances that do not appear as particles. This technique can be used after the oil has been removed from the water," says Thor Thorsen, senior scientist at SINTEF Applied Chemistry. The project is based on this Division´s previous experience of purifying water and gas.
"Cleaning produced water is a real challenge. There is a long list of substances which we have to try to remove. We will try out different technologies which will have to work together, since we realise that we can hardly expect a single method to deal with all the substances that are found in produced water," says Thorsen.
Ion exchange
Heavy metals can be removed by means of the ion exchange method of water purification. The basic technology behind this method has been well tested at SINTEF NHL."We will carry out tests to see how ion exchange functions vis-à-vis heavy metals, and in combination with other purification processes. This will be done in the new test rig that is to be built for purification of dissolved components," says chief scientist Bjørnar Eikebrokk at SINTEF NHL.
The principle of ion exchange is that heavy metals consist of ions - i.e. molecules with a positive electrical surface charge. The water flows across a filter bed of small porous particles that contain positively charged exchangeable ions. The molecules force their way into the porous particles and their ions change place with the ions in the particles, with the result that heavy metal molecules are stuck in the filter bed. When all the exchangeable ions have been "used up" or exchanged, the filter bed must be recharged or regenerated via an acid bath, which supplies it with new exchangeable ions.